I just checked, rotating a rotated rectangle does not change anything
but the angle in the Graphic Details drawer.
And the width and height of the rotated rectangle are shown exact in
Graphic Details. That provides an exact measurement.
Are you using the Rotated Rectangle Tool - the one near the bottom of
the main tool palette. - use that. or if you draw a normal rectangle
and rotate it with the rotate tool it turns into a proper rotated
rectangle.
If still trouble send in the drawing to support.